It is well known in the art to employ motor operated appliances for various tasks involved in preparing food, including slicing, shredding, mixing, kneading, chopping and the like. One of the problems involved in using such appliances was the difficulty, particularly with appliances adapted to mix and knead, in maintaining a constant mixing or kneading velocity of the mixing or kneading elements as the load on the appliance changed.
A number of solutions were found to the problem. One of them involved the use of various closed loop speed control systems, some of which employed a mechanical governor as a speed sensor which controlled a firing angle of a triac, such as disclosed in U.S. Pat. No. 4,227,128 to Cockroft et al. Another approach involved the use of an all electronic speed control which employed a magnetic tachometer to sense a rotational speed of an interrupter wheel mechanically connected to a motor and which controlled the firing angle of a triac connected to the motor, as is disclosed in U.S. Pat. No. 4,326,153 to Contri.
Despite the advantages which clearly accrued from these prior art speed control systems, there were still problems in the use of such systems. The primary problem involved the difficulty when such a system was used with a mixing appliance, such as an electric food mixer of the type disclosed in U.S. Pat. No. 4,277,181 to Stahly et al. of introducing powders or liquid ingredients into a mixing bowl while the mixing appliance was running in accordance with the dictates of a recipe. Users often found that when powders, in particular, were introduced into the mixing bowl they had a tendency to puff out or be thrown out of the bowl by the whippers, causing a loss of the premeasured ingredients which might alter the characteristics of the food being mixed and also lead to the possibility that some of the ejected powder, in airborne form, could be drawn in through cooling vents in the mixer, ultimately collecting therein and damaging the mixer. Similar problems were encountered with the addition of various liquids which could be splashed out of the mixing bowl due to the rapid rotation of the whippers in accordance with recipe instructions.
One possible solution for this problem would be the manual adjustment of the mixer to a lower speed while the powders and liquid are being introduced into the bowl. A problem arises, however, in that users often forget to return the mixer to the desired mixing speed once the powders or liquid have been introduced. This can lead to damage to the mixer due to the fact that prolonged low speed operation of mixers employing universal motors under relatively high torque loads can cause overheating and motor damage.
What is needed, then, is a food preparation or mixing appliance having the ability to temporarily reduce its speed to allow the introduction of powders or liquids into a mixing bowl without subsequently ejecting them therefrom and thereafter be able to return automatically to the preselected desired mixing speed.